ON THE CONSERVATION OF FORCE 209 



forms work, and this work is produced at the cost of its 

 heat. Hence the cooling. If, on the contrary, the gas is 

 suddenly allowed to issue into a perfectly exhausted space 

 where it finds no resistance, it does not become cool, as 

 Joule has shown; or if individual parts of it become cool, 

 others become warm ; and, after the temperature has become 

 equalised, this is exactly as much as before the sudden ex- 

 pansion of the gaseous mass. 



How much heat the various gases disengage when they 

 are compressed, and how much work is necessary for their 

 compression; or, conversely, how much heat disappears 

 when they expand under a pressure equal to their own 

 counterpressure, was partly known from the older physical 

 experiments, and has partly been determined by the recent 

 experiments of Regnault by extremely perfect methods. Cal- 

 culations with the best data of this kind give us the value 

 of the thermal equivalent from experiments : 



With atmospheric air 426.0 metres 



" oxygen 425-7 



" nitrogen * .... 431.3 



" hydrogen 425.3 " 



Comparing these numbers with those which determine the 

 equivalence of heat and mechanical work in friction, as 

 close an agreement is seen as can at all be expected from 

 numbers which have been obtained by such varied investi- 

 gations of different observers. 



Thus then : a certain quantity of heat may be changed into 

 a definite quantity of work; this quantity of work can also 

 be retrans formed into heat, and, indeed, into exactly the 

 same quantity of heat as that from which it originated; in 

 a mechanical point of view, they are exactly equivalent. 

 Heat is a new form in which a quantity of work may appear. 



These facts no longer permit us to regard heat as a sub- 

 stance, for its quantity is not unchangeable. It can be pro- 

 duced anew from the vis viva of motion destroyed; it can 

 be destroyed, and then produces motion. We must rather 

 conclude from this that heat itself is a motion, an internal 

 invisible motion of the smallest elementary particles of 

 bodies. If, therefore, motion seems lost in friction and 

 impact, it is not actually lost, but only passes from the great 



